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Energy, Molecules and Chemical Reactions

Energy, potential energy, is stored in the covalent bonds holding atoms together in the form of molecules. This is often called chemical energy. Except at absolute zero (the coldest temperature it is possible to reach), all molecules move. This movement is a form of kinetic energy, and the more the molecules move the more kinetic energy they have. Molecules in solids don't move much, they just vibrate. Molecules in liquids move faster and further, but they stick together enough to hold them in a small volume - the liquid. In a gas, however, the molecules are moving fast (over 1,000 miles per hour), moving in all directions, and don't stay together so they spread out and fill the available volume.

When two molecules bump into one another two things can happen. If they don't bump too hard they will behave like small solid spheres and just bounce off, each going in its own new direction like the balls on a pool table. Some kinetic energy may be exchanged, but the total energy will stay the same.

But, if two molecules bump into each other with enough force, something very dramatic takes place. The bonds holding the atoms absorb some of the kinetic energy and they break apart. With no forces holding them together the atoms separate, and for a tiny, tiny fraction of the tiniest part of a second, the atoms are 'free' of each other. This is a very unstable state and one that is corrected at once.

Almost immediately the 'free' atoms seek new partners and start forming new covalent bonds. New and different combinations of atoms in new permutations of bonds form new and different molecules. Once reformed the new molecules move apart and travel with kinetic energy onto their next meeting. A chemical reaction has taken place.

The molecules that bump into one another in the first place are called the reacting molecules or reactants in a chemical reaction. The new molecules that are formed after the rearrangement are called the products. A typical chemical reaction involves both reactants and products.

As a natural process, chemical reactions have to obey both the laws of thermodynamics. The total amount of energy at the beginning of a chemical reaction must be the same as the total amount of energy at the end of the chemical reaction (first law). But the second law says that the available, useable energy must be less and the entropy must be higher. How?

Different covalent bonds hold different amounts of energy; some higher amounts some lower amounts. If, during a chemical reaction, two 'high energy' covalent bonds are broken, the new ones that form must be at a lower level of energy. For example when molecular hydrogen and molecular oxygen react together to form water, the energy stored in the covalent bonds of the hydrogen and oxygen molecules is higher than that found in the hydrogen-oxygen bonds of water. Some of the 'extra' energy that is lost as this happens becomes kinetic energy and the water molecules move faster and further. We say that the liquid becomes 'hot'.

During chemical reactions of this type, therefore, potential chemical energy in the bonds holding the reacting molecules together, become lower energy covalent bonds in the product molecules (the amount of available energy is decreased), and at the same time the product molecules have more kinetic energy and move faster. This is unavailable energy, increasing the disorder and increasing the entropy.

The second law of thermodynamics has been obeyed.


Science@a Distance
© 2001, Professor John Blamire